This section provides background information related to the present disclosure which is not necessarily prior art.
Operating temperatures for electrical devices, including appliances, electronics, motors and the like typically have an optimum or preferred range, above which damage can occur to the device or its components, or safely operating the device becomes a concern. Various known devices are capable of protecting against over-temperature conditions by interrupting the electrical current in the device.
One device particularly suitable for over-temperature protection and current interruption is known as a thermal cut-off (TCO) device. A TCO device is typically installed in an electrical application between the current source and electrical components, such that the TCO device is capable of interrupting the circuit continuity in or to a device in the event of an undesirable over-temperature condition. TCO devices are often designed to shut off the flow of electric current to the application in an irreversible manner, without the option of resetting the TCO device current interrupting device.
An exemplary TCO device known in the art is illustrated in FIG. 1. In general, the TCO device 100 includes a conductive metallic case or housing 102 having a first electrical conductor or lead 104 in electrical contact with a closed first end 106 of the case 102. An isolation bushing 108, such as a ceramic bushing, is disposed in an opening of the case 102. The case 102 further includes a retainer edge 110, which secures the isolation bushing 108 within a second end 112 of the case 102. A second electrical conductor or isolated lead 116 is at least partially disposed within the case 102 through an opening 118 in the second end 112 of the case 102. The second electrical conductor 116 passes through the isolation bushing 108 and has an enlarged distal end 120 disposed against one side of the isolation bushing 108 and a second end 122 projecting out of the outer end of the isolation bushing 108. A seal 124 is disposed over the opening 118 and can create sealing contact with the case 102, the isolation bushing 108, and the exposed portion of the second end 122 of the second electrical conductor 116. In this manner, an interior portion of the case 102 is substantially sealed from the external environment.
An electric current interruption assembly 114 for actuating the device in response to a high temperature, for example, is generally disposed between the first and second electrical conductors. The current interruption assembly 114 actuates or “trips” to break the continuity of an electric circuit through the TCO device 100. The current interruption assembly includes a moveable, sliding contact member 126 formed of electrically conductive material, such as a metal. The sliding contact member 126 is disposed inside the case 102 and is disposed in peripheral sliding engagement with the internal surface of the case 102 to provide electrical contact therebetween. Moreover, when the TCO device is operating at a temperature that is below its predetermined threshold set-point temperature, the sliding contact member 126 is disposed in electrical contact with the distal end 120 of the second electrical conductor 116.
The current interruption assembly 114 also includes a biasing means. The biasing means biases the sliding contact member 126 against the distal end 120 of the second electrical conductor 116 to establish electrical contact in a first operating condition where operating temperatures are below the threshold set-point temperature of the TCO device. As shown in the Figures, the biasing means includes first and second compression springs 128, 130, each having a different spring rate, which are respectively disposed on opposite sides of the sliding contact member 126. Two disk members 131, 133 are disposed on opposite sides of the first compression spring 128. The disk members act to substantially evenly distribute the bias of the first compression spring 128.
Also included in the current interruption assembly 114 is a thermally responsive member 132 which, when in a solid physical state, can take the form of a pellet. The solid thermally responsive member 132 is disposed in the case 102 and occupies a volume at the first end 106. The first compression spring 128 of the current interruption assembly 114 is disposed between the thermally responsive member 132 and the sliding contact member 126 and biases the sliding contact member 126 toward engagement with the second electrical conductor 116. The second compression spring 130 is disposed between the sliding contact member 126 and the isolation bushing 108 and biases the sliding contact member 126 away from engagement with the second electrical conductor 116. Because the first compression spring 128 has a greater bias than the second compression spring 130, a net force acts against the sliding contact member 126 to urge the sliding contact member 126 into contact and electrical engagement with the enlarged distal end 120 of the second electrical conductor 116. In this manner, an electrical circuit is established through the TCO device by the first electrical conductor 104, through the electrically conductive case 102, to the sliding contact member 126, and to the second electrical conductor 116.
The thermally responsive member 132 has a reliably stable solid phase at a first operating condition where the operating temperature of the device in which the TCO device is incorporated or the temperature of the surrounding environment, for example, is below a predetermined threshold set-point temperature. The solid thermally responsive member 132, however, reliably transitions to a different physical state when the operating temperature meets or exceeds the threshold set-point temperature in a second operating condition. Under such conditions, the thermally responsive member, e.g., melts, liquefies, softens, volatilizes, or otherwise transitions to a different physical state such that it cannot oppose the force of the biasing means.
With further reference to FIG. 2, a portion of the second electrical conductor 116 is illustrated in greater detail. The second electrical conductor 116 includes a shaft portion 134 terminating at the distal end 120. The distal end 120 has a contact surface 138 at one end and a shoulder 140 at an opposite end adjacent to the shaft portion 134. Referring again to FIG. 1, the distal end 120 of the second electrical conductor 116 abuts the sliding contact member 126 at the contact surface 138 to close the electric circuit through the TCO under conditions when operating temperatures are below the threshold set-point temperature of the TCO device. The contact surface 138 has a convex, hemispherical shape such that only the most distal portion of distal end 120 of the second electrical conductor 116 comes into contact with the sliding contact member 126 to close the electric circuit. Consequently, even under the best of circumstances, only a very small surface area of the second electrical conductor 116 and the sliding contact member 126 engage to close the electric circuit.
Under conditions where the operating or ambient temperature meets or exceeds the TCO device's threshold set-point temperature, the thermal pellet transitions to a different physical state such that it no longer occupies the volume at the first end 106 of the case 102. As such, the first compression spring 128 expands to occupy the space formerly occupied by the thermal pellet 132. In doing so, the first compression spring 128 no longer biases the sliding contact member 126 into engagement with the second electrical conductor 116 with enough force to overcome the bias of the second compression spring 130. Consequently, the bias of the second compression spring 130 forces the sliding contact member 126 out of engagement with the second electrical conductor 116, thereby interrupting the electric circuit in the TCO device.
TCO devices are known to have an element of self-heating (I2R heating) when they carry electrical current. A reduction in this self-heating would improve the TCO device's operating by allowing it to run at a cooler temperature away from the TCO device's threshold set-point temperature and the phase transition temperature of the thermal pellet. Also, the continued evolution of the TCO device's design requires changes in its construction, such as material options, plating thicknesses, contact systems, etc. In several instances, prior attempts to change these features to improve the TCO device have resulted in unfavorable shifts in the performance of the TCO device.